Hard clipping diodes. It uses MOSFETs, but they're just acting as silicon diodes in that case because they didn't use a series diode to block the reverse conduction to use only the DG-S junction (which now does nothing because the silicon diode going the other way is now conducting at a much lower Fv). The parallel resistor is there to attenuate the signal level to keep it from clipping as much with a typical line level.

You might get some decent sounds out of the transformers, but it's silly that they advertise these as transformer-involved anything since almost all the lifting is done by the diodes. It's probably just an inexpensive transformer with poor frequency response -- so you can tell it's in the circuit but you're probably not saturating it much if at all using these.

I'm not suggesting that they sound bad or anything, only that the transformers might be added cost that, strictly speaking, might not need to be there.

I was just looking at these the other day! Would love to see them ported over to the stomp box world. At the moment the 15ips has me thinking it could be great in a modded Boss DD-3 as a feedback loop insert.